In the recent twenty years, the power electronic technology has obtained a quickly development, and has been widely applied to the fields of electrical power, chemical engineering and communications. The electrical power apparatuses are mostly going through the rectifiers and the electrical power network interfaces, a typical rectifier is a nonlinear circuit including diodes or thyristors, and the nonlinear circuit generates lots of current harmonics and reactive powers in the electrical power network, pollutes the electrical power network, and becomes a public nuisance of the electrical power. The electrical power apparatuses have become the main harmonic sources of the electrical power network. The main method to restrain the harmonics is the active approach, i.e. designing a new generation of high-performance rectifiers with features of having sinusoidal input current, containing low amount of harmonics and having high power factor, namely, it has the power factor correction function. Recently, the PFC circuits have attained a great development, and have become an important research direction of the power electronics.
The single-phase PFC technology currently approaches increasingly mature in circuit topology and in control, and the frequently used single-phase PFC circuit is the boost circuit. It has the advantages of having simple configuration, requiring smaller EMI filter etc. But this kind of configuration could only apply to the occasions while the output voltage is larger than the peak value of the input voltage. For the input voltage having a broad range, sometimes the input voltage is higher than the output voltage, namely the input voltage needs to be decreased and guaranteed that the input current tracks the input voltage nicely so as to gain a lower THD. At the moment, the boost circuit could not accomplish this function, and a buck configuration is used for this occasion. As shown in FIG. 1, it is a topology of a single-phase buck-boost PFC circuit applied to a broad range of input voltage in the prior art. It has diodes B1-B4 and D1-D2, switches S1-S2, inductor L1, input power source Vin and output capacitor C1, and outputs a voltage Vo.
The working modes of this kind of conversion circuit are as follows:Vo>√{square root over (2)}Vin   a.
Wherein, Vo is the output voltage, and Vin is the input voltage. Under this kind of operation conditions, a waveform diagram of the input voltage Vin and the output voltage Vo is shown in FIG. 2, the output voltage is always higher than the input voltage, the converter must operate under the boost mode, switch S1 is turned on, diode D1 is turned off, and under this circumstances, the converter is the conventional boost PFC circuit.Vo≦√{square root over (2)}Vin   b.
It is easy to find from FIG. 3, the converter operates under the buck and the boost working modes. Between periods π−α and π+α, the output voltage is larger than the input voltage, switch S1 is always turned on, D1 is always turned off, and the converter works under the boost mode. Between periods α and π−α, the output voltage is smaller than the input voltage, switch S2 is always turned off, D2 is always turned on, and the converter works under the buck mode.
This kind of circuit is only suitable for the condition of single-phase input, for certain occasions, we need to use the three-phase input voltage, and thus this kind of single-phase circuit could not fulfill the requirements of the system. In the three-phase input voltage application occasions, there are many other conventional methods used to decrease the THD of the input current. A relatively frequently used method is shown in FIG. 4. In which, it includes diodes, D1-D14, capacitor Co and C1-C3, switches S1-S4, inductors L1-L5 and AC power sources Vi1-Vi3.
The topology of FIG. 4 could be divided into two parts: the front part is the input buck part (buck input stage), and the rear part is the output boost part (boost output stage). This PFC circuit could be employed in rectification for the three-phase three-line configuration, has a simple configuration, and has relatively less elements. But it also has drawbacks: due to that the neutral point of the three-phase three-line configuration is formed by the three-phase AC input voltage connecting to three capacitors, which is not the absolute zero potential point, in the three-phase three-line condition, the three-phase inputs are coupled to each other, thus the three-phase current control is relatively harder and the THD is relatively higher. This kind of topology has a lower efficiency due to that the current flows through a higher amount of elements. Especially, due to that the three-phase inputs are electrically coupled, this system can not be parallel-connected unless a transformer is used for isolation. However, if it is parallel-connected, the current of one phase will become a reverse-current of another phase, which will result in unbalance among circuit currents of different phases, and it is difficult to make redundant system with high reliability.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a three-phase buck-boost PFC circuit and a controlling method thereof.